Patch Target: A tiny library for creating valid unittest.mock.patch target arguments

The ability to monkey patch modules, functions, classes, and class instances is a powerful part of testing Python code; however, the unittest.mock.patch function depends on the developer providing a precise module path AS A STRING to the object to be patched. If you refactor where your modules live or make other changes, you will find the annoying and hard-to-decipher-at-first errors pointing to an invalid module path string.

This small library, consisting of a single exported function, attempts to make using this function a more straightforward, reliably correct experience.

the patch_target function takes 2 arguments:

• a host_module of type ModuleType
• and an object_to_be_patched, which can be of type Any, but note that the only Patchable types are Named | str, where Named is a protocol describing any object with a __name__ attr
• see note* on passing str types below

Since you're dealing with python objects instead of strings, you get more guarantees out of the box. E.g. since you have to pass in a module instead of a string, that means you have to have successfully imported the module into your test to begin with

example:

Given a src code file like this:

# myapp.mymodule.mysubmodule

import datetime
from uuid import uuid4

my_module_level_attribute = "some_value"

def get_current_time():
    return datetime.datetime.now()

def generate_new_id():
    return uuid4()

You can patch the non-deterministic pieces (current time and uuid generation) like so:

from unittest.mock import patch, Mock
from myapp.my_module import my_submodule  # noqa
import datetime

import uuid

from patch_target import patch_target


# using the patch decorator to override a module imported into the host_module
@patch(patch_target(my_submodule, datetime))   # Using string paths the patch arg would be  "myapp.mymodule.my_submodule.datetime"
def test_get_current_time(mock_datetime: Mock) -> None:
    the_beginning_of_time = datetime.datetime(1970, 1, 1)
    mock_datetime.datetime.now.return_value = the_beginning_of_time
    
    actual = my_submodule.get_current_time()
    expected = the_beginning_of_time

    assert actual == expected


# using the patch context manager override a function imported into the host_module
def test_generate_new_id() -> None:
    fake_id = "my-super-cool-id"
    with patch(patch_target(my_submodule, uuid.uuid4)) as mock_uuid4: # Using string paths the patch arg would be  "myapp.mymodule.my_submodule.uuid.uuid4"
        mock_uuid4.return_value = fake_id
        
        actual = my_submodule.generate_new_id()
        expected = fake_id
        
        assert actual == expected


# using the patch context manager override a module_attribute defined in the host_module
def test_get_module_level_attribute() -> None:
    with patch(patch_target(my_submodule, "my_module_level_attribute"), "some_other_value"): # Using string paths the patch arg would be  "myapp.mymodule.my_submodule.my_module_level_attribute"        
        actual = my_submodule.my_module_level_attribute
        expected = "some_other_value"
        
        assert actual == expected

* Note that though patching module-level attributes using the string name of the variable is supported, you can just use unittest.mock.patch.object the same way. So you don't need this lib to accomplish it.

# using unittest.mock.patch.object
from unittest.mock import patch

from myapp.my_module import my_submodule  # noqa

def test_get_module_level_attribute() -> None:
    with patch.object(my_submodule, "my_module_level_attribute", "some_other_value"): # Using string paths the patch arg would be  "myapp.mymodule.my_submodule.my_module_level_attribute"        
        actual = my_submodule.my_module_level_attribute
        expected = "some_other_value"
        
        assert actual == expected